Also titled "The Room With Four Walls" or "The Silent Room." Chapter two of Souichi's Diary of Curses.
While trying to study for exams, Kouichi is constantly disturbed by mysterious noises and events. He thinks Souichi is responsible, but each time it can't be proven that Souichi was there. Souichi himself is delighted and says the house must have a poltergeist. Kouichi eventually does catch Souichi bothering him, but Souichi says he was just trying to "help" the ghost.
Mr. Tsujii comes up with the idea that they get his friend, a carpenter, to sound-proof Kouichi's room. Unfortunately his friend is busy and cannot start work for another month, too late for Kouichi's exams. The following day a strange, corpse-like man named Mr. Tagaisu arrives at the house. He says that the construction company sent him to work on Kouichi's room. He says that the walls will need four levels of insulation because the word for four sounds like the word for death, thus four levels will "kill" the noise. Souichi interrupts the construction work by hammering his own nails into the wall, but Tagaisu is impressed at Souichi's technique and asks him to help out.
Kouichi returns home to find that his windows have been completely sealed off. Tagaisu says this was necessary to prevent sound from coming in; and, satisfied with his work, he leaves. Kouichi discovers that Tagaisu has actually installed four sets of walls inside the room, each with their own door, reducing the room to a very tiny space. Although unhappy, Kouichi decides that at least he's got a quiet space to study.
However, Kouichi is disturbed by what sounds like a cricket chirping. Because the room is soundproof, he works out that Souichi must be inside one of the spaces and making the noise. He catches Souichi inside one of the gaps and chases him, but is badly injured by nails that Souichi put there, and is eventually lured into falling into the septic tank. We are told that Souichi stayed in the enclosed space waiting for Kouichi to return. Eventually, he almost suffocated and had to be rescued.
Mr. Tsujii calls the construction company to complain but is told they did not send Mr. Tagaisu and have never heard of him. Kouichi wonders who Tagaisu could have been.